The fabrication of modern, high-speed microelectronic devices includes a number of intricate and costly processing and fabrication steps which are conducted on a very small, microscopic scale. Various types of such microelectronic devices, as well as microelectronic integrated circuits (IC's) which incorporate as many as thousands of such devices, are fabricated and mass produced on silicon wafers. Each silicon wafer generally comprises an array of numerous electrically-isolated individual integrated microelectronic circuits. Each individual circuit on the wafer typically has numerous fabricated pads which are located proximate to the bulk of the circuit and which are electrically connected to the individual circuit itself. The pads serve as electrical interfaces for routing electrical current through the individual microelectronic circuit. Once a silicon wafer and the array of circuits embedded thereon is completely fabricated, the wafer is then generally sliced apart to thereby physically separate the array of circuits into individual circuits. Each individual circuit on its separated portion of the previously-whole silicon wafer is generically referred to as a "chip." In this separated chip form, an individual circuit can then be wire-bonded into, for example, a plastic or ceramic package and sealed therein for general electrical and thermal insulation purposes. The pads of the packaged chip are commonly wirebonded to electrical leads mounted on the outside of the package to provide electrical access to the packaged chip via the external leads.
Presently, free-standing micro-structure devices called MEMS (Micro-Electrical Mechanical Systems) are gaining ever-growing popularity in the microelectronics industry. Such MEMS devices may include, for example, a micro-accelerometer, a micro-mechanical filter, a pressure sensor, a gyroscope, or a micro-resonator. In light of such popularity, manufacturers are now attempting to fabricate composite devices which integrate both microelectronic integrated circuits and MEMS together on the same chip. However, due to the unique nature of MEMS devices, more reliable packaging processes and methods need to be developed to simultaneously accommodate both microelectronic integrated circuits and MEMS so that composite devices can be mass-produced commercially.
More particularly, many MEMS devices, by their very nature, must be encapsulated and hermetically sealed within a microshell in order to operate properly. For example, a MEMS device such as a pressure sensor or an accelerometer has movable micro-mechanical parts which must be permitted to move for the MEMS to operate properly. Such MEMS require hermetic encapsulation within a microshell to prevent contaminants, such as dust, from interfering with the MEMS device performance. Furthermore, due to the fact that a typical MEMS device has a size on the order of 10.sup.-6 to 10.sup.-3 meter, the fabrication and precise positioning of a microshell over a MEMS device to thereby encapsulate the device can prove to be a significant challenge, for such a small scale environment can require microfabrication and precision positioning on the order of 10.sup.-7 to 10.sup.-9 meter.
One prior art process method for hermetically encapsulating and thereby protecting a MEMS device (in this instance, a microresonator) is described in U.S. Pat. No. 5,589,082, incorporated herein in its entirety by reference (see FIG. 1). The initial steps in the process include standard surface micromachining steps which ultimately produce a comb-shaped resonator MEMS device mounted on a silicon wafer substrate (see FIG. 1(a)). Since the fingers of the comb-shaped resonator must be protected so that they are free to move to help operate the resonator correctly, a hermetically-sealed microshell needs to be formed about the resonator. Thus, next, a thick layer of PSG (phosphorus doped glass) is deposited on the silicon substrate so that the PSG surrounds and covers the MEMS device, thereby defining the area to be sealed by a microshell (see FIG. 1(b)). Then, a thin layer of PSG is deposited, patterned, and etched to form etch channels (see FIG. 1(c)). Next, a thin layer silicon nitride is deposited over the thick layer of PSG to define a microshell, and etch holes are thereafter precisely defined in the thin layer of silicon nitride (see FIG. 1(d)). Thereafter, all PSG within the microshell is etched away in a concentrated HF gas bath via the etch holes. After rinsing in water and in methanol, the silicon wafer and its encapsulated MEMS device is dried using a supercritical CO2 process. Finally, a relatively thin layer of nitride is deposited to thereby hermetically seal the MEMS resonator device within its microshell, and contact pads are thereafter etched open to provide electrical access to the MEMS device (see FIG. 1(e)). In this manner, the MEMS device is protected from contaminants, and electrical access is also provided.
This particular MEMS encapsulation process method, as briefly described hereinabove and more fully explained in U.S. Pat. No. 5,589,082, is operable for the very narrow purpose intended, but a simpler and more versatile process method is instead desired. In particular, a process method which hermetically encapsulizes a MEMS device while at the same time is highly compatible with standard process methods used to fabricate microelectronic integrated circuits is highly desirable, for such would enable composite devices to be mass-produced commercially.
Another prior art process method for encapsulating a MEMS device is described in U.S. Pat. No. 5,576,251, incorporated herein in its entirety by reference, wherein two substrates are fused together to form a protective covering for a MEMS device. A bonding material is interposed between the two substrates, and the temperature of the bonding material is raised to about 950.degree. C. for about 30 minutes during which time fusion bonding of the two substrates occurs, and chemical reactions remove gas from the cavity between the two substrates, thereby creating a vacuum and a hermetically sealed enclosure about the MEMS device. In another prior art process method, described in U.S. Pat. No. 5,668,033, global heating is utilized to bond two substrates together within an oven. In this particular method, the entire MEMS structure must be heated in an oven to a temperature sufficient to form the bond between the two substrates. Such a temperature, however, often has undesirable and damaging effects on the MEMS device. In still another prior art process method, described in U.S. Pat. No. 4,625,561, the method therein also teaches bonding by global heating in a furnace. One common aspect of the particular prior art encapsulation and bonding methods alluded to hereinabove is that a high temperature is required to facilitate the process of bonding. As a result, such methods are not suitable for use in situations involving microelectronic integrated circuits and)/or MEMS which cannot tolerate exposure to such high temperatures.
In general, micromachining and microfabrication process techniques which are typically used to produce MEMS devices and/or microelectronic integrated circuits are very costly. In addition to being costly to produce, MEMS devices and microelectronic integrated circuits are typically temperature sensitive and can be permanently damaged if exposed to high temperatures. Thus, the integration of a MEMS device with a microelectronic integrated circuit device must be carefully executed so as to not permanently damage any expensive temperature-sensitive device. In light of such, a common critical concern in fabricating a composite device is how to integrate a free-standing MEMS device, which requires a high-temperature bonding process for encapsulation, with a temperature-sensitive microelectronic integrated circuit.
Bonding techniques, including fusion bonding (such as silicon-to-silicon bonding), eutectic bonding (such as silicon-to-gold bonding), and anodic bonding (such as silicon-to-glass bonding), have all been used both in microelectronic integrated circuit and MEMS fabrication for many years. Each of these different bonding processes require two basic elements in order to be successful. First, the two surfaces to be bonded must each be flat to ensure intimate contact for proper bonding. Second, proper processing temperatures are required to provide the proper bonding energy. For example, a conventional silicon-to-silicon fusion bonding process occurs at a bonding temperature of above 1,000.degree. C. On the other hand, anodic bonding is performed at a lower temperature of about 450.degree. C., but requires the assistance of a high electrostatic field. Silicon-gold eutectic bonding theoretically occurs at a temperature of 363.degree. C. Among these bonding processes, one common drawback is the high temperature requirement that may damage and degrade temperature-sensitive integrated circuits and/or MEMS. Therefore, such bonding processes are not generally applicable in fabricating or packaging devices when temperature-sensitive devices are involved. For the past few years, many efforts have been undertaken to find a reliable bonding process that can be conducted at a low temperature. Unfortunately, the success of each of these bonding processes depends highly upon a particular fabrication process' idiosyncracies, such as the particular bonding material used, surface treatment, and the flatness of the actual surfaces which are to be bonded together. Due to the significant number of such contingencies, such bonding processes are generally neither adaptable nor economical for the large scale production of composite devices.
Therefore, presently known high-temperature bonding process techniques, for encapsulating a MEMS device under a microshell, are highly process dependent and are generally not suitable for producing composite devices which incorporate temperature-sensitive microelectronic integrated circuits, for such bonding techniques require high temperatures which may damage temperature-sensitive integrated circuits and/or MEMS devices. In addition, such bonding processes typically strictly require very flat bonding surfaces, select bonding materials, and very tight process control to be successful. As a result, presently known bonding processes are not ideal for encapsulating free-standing MEMS structures on composite devices.